As shown in FIG. 4, in case of forming a contact hole 20 by a plasma etching through an insulating film layer 16, which covers a gate 12 formed on a semiconductor substrate 10 such as a silicon substrate, and is made of a silicon oxide film such as SiO2, a self-alignment contact technology 20 may be applied. In the self-alignment contact technology, the contact hole 20 is formed in a self-aligning manner in a small and compact area between gates 12 while a protective film layer 14 such as a silicon nitride (SiN) film is formed on each gate 12, thereby preventing the gate 12 from being etched in the course of forming the contact hole 20.
When the contact hole 20 is formed by employing the self-alignment contact technology, there is used a CF-based gas, such as C4F8, as a processing gas in case of processing the plasma etching, and a gaseous mixture including O2 is used as an etching gas for removing deposits. With a recent improvement in the integration of the semiconductor devices and increased demands for a miniaturization of various elements formed on a semiconductor substrate, the design rule has become even finer. Consequently, it has become necessary to reduce a gap between gates (electrodes) formed on the semiconductor substrate and thus a high aspect ratio of a contact hole formed between the gates (electrodes) is required. However, the plasma etching employing the conventional processing gases as described above is disadvantageous in that as the aspect ratio of the contact hole becomes higher, because of the narrow gaps between the gates, the etching time becomes longer to prevent degradation of the capability to pierce an etching target and occurrence of an etching stop. However, as shown in FIG. 4, a large portion of a shoulder (edge) 14a of the silicon nitride film, acting as a protective film layer formed on a surface of the gate 12, is extended into an inner space of the contact hole 20 to be formed through the silicon oxide film acting as the insulating film layer 16. Therefore, the shoulder 14a may be very readily etched. Furthermore, the shoulder (edge) 14a of the protective film layer 14 may be severely etched depending on etching selectivity of the insulating film layer 16 over the protective film layer 14 of the gate 12, thereby rendering the gate 12 undesirably exposed thereto.